Many commercial solid products include a color measurement in their specifications. Indeed, with many light-colored solid products, generally employed as intermediates, the color specification is of critical importance. This is true of both organic and inorganic materials. As one example, the shipping specification for purified terephthalic acid (PTA) is a color measurement.
In light-colored, or white, solids, such as PTA, the degree of yellowness is generally taken as a measurement of the degree of impurity. There is employed, as a measure of yellowness, a function characterized as the b-value, generally described in Hunter, Richard S., The Measurement of Appearance, John Wiley & Son (New York), 1975, pages 122-123. The b-value typically has a maximum value at a wavelength of about 450 nanometers (nm) and a half width of about 60 nm. In the case of PTA, variations in the b-value are typically less than the human eye can detect, thus requiring very sophisticated reflectance spectrophotometric equipment for its measurement. This is a time-consuming procedure, poorly adapted to use with a continuous non-homogeneous production line for a critical product. Typically, thousands of pounds of product are prepared during the interval between analyses.
The sensor apparatus of this invention is capable of being inserted directly into a product outlet (e.g., dryer) line to provide a continuous on-line determination of the b-value, coupled with an ability to measure other selected wavelengths of scattered light.
Reflectance measurements are limited in that there is no simple method for increasing sensitivity other than developing equipment capable of measuring reflected light intensity to more significant figures. The illustration presented in FIG. 1 shows the development of reflectance spectra in light-scattering systems. Deeper penetration of light, providing multiple reflections, generates weaker signals which are, at the same time, preferred because they are more precise in the information they can afford. Again, the sensor apparatus of this invention can readily be adapted for increased sensitivity by increasing the distance between the source of light scattering and the point of analysis.
An extensive discussion of reflectance analysis employing scattered light in the near-infrared has been published in Analytical Chemistry, vol. 55, p. 1165A (1983).
Means generally available for the requisite light transmission for utilizing reflectance spectra have been both inconvenient as to size and inefficient as to loss of signal intensity. While not required for the practice of this invention, the use of fiber optics greatly improves the sensor efficiency and the convenience of its application.
Fiber optic cables have received great attention and development in the broad field of communications. Other adaptations having significant utility have also been developed.
For example, the use of fiber optics in apparatus for measuring airborne solids during environmental surveillance is described in U.S. Pat. No. 4,459,024. Fiberoptic cables are employed in pairs to provide light output and to receive scattered light from particles of airborne pollution. Finally, an information processor is employed to evaluate the scattered light signals.
Recently, a fiber optic probe and transmission system has been employed with a scanning spectrophotometer, as described in Chemical Engineering, Dec. 9/23 (1985), p. 37. This system employs light having wavelengths varying from the ultraviolet (250 nm) to the near-infrared (2,200 nm).
Although the methods and instrumentation presently known in the art find widely differing usages, there is a need for an analytical unit having utility in accurately determining trace impurity levels in a broad range of light-scattering media, particularly those which are non-homogeneous with respect both to space and time.